Sensor is a device that transforms a measured quantity into a readable format, typically into an electrical signal. Nowadays, there are commercially available sensors virtually for any measurement purpose. According to the connectivity, sensors can be divided into wireless and wired sensors. Wired sensors are connected via wiring harnesses or cable assemblies to a reader device. Wireless sensors can be read without a physical connection to the sensor, and are often realized equipping the sensor with a radio transceiver. The transmitted radio signal is interpreted by a receiver which converts the wireless signal into a desired output. Wireless operation can be bene cial in many applications, where wired connection is di cult for example due to harsh operating conditions (like temperature and pressure), rotating parts, or cost and complexity of wiring. However, wireless sensors also have some drawbacks such as limited lifetime due to battery, limited read-out distance due to attenuation and interference, security issues because of the uncontrollable propagation of the signal and potentially low speed of communication. Based on the power source and communication principle, wireless sensors can be divided into three categories: active sensors, semi-passive sensors and passive sensors.
Active wireless sensors usually have both a radio transceiver and an on-board battery that is used to power up the transceiver. Active wireless sensors, having their own power sources, can use powerful transmitters and sensitive receivers. However, the battery on board limits the life time and also increases the size and weight. Due to more complex circuit, the price of an active sensor can be much higher than that of a passive sensor.
Semi-passive wireless sensors do not contain a radio transceiver, but are equipped with a battery. The battery is used to power up an integrated circuitry (IC) and enables the sensors to operate independently of the reader device or to maintain memory in the sensor. Semi-passive battery-assisted sensors utilize modulated backscattering technique for communication. This means that semi-passive sensors do not require any power from the on-board battery for transmission, but the sensor simply reflects back some of the power emitted by the reader device.
Unlike the active and semi-passive sensors, passive sensors do not require an on-board battery. Therefore they can be less complex, smaller, more inexpensive, and their lifetime is not limited by the power supply. The typical read-out distance of passive wireless sensors is between 10 cm and 3 m. Passive wireless sensors can be divided into four main categories: radio frequency identification (RFID) tags, electrical resonance circuit sensors, surface acoustic wave (SAW), harmonic sensors and intermodulation sensors.
RFID is an identification technology that uses radio waves to communicate between tags and a reader and it is used to identify items. There are a few advantages of RFID over optical barcode identification such as no line-of-sight is required between the reader device and the tag, and the RFID reader can also read hundreds of tags at a time. Passive RFID tags utilize the modulated backscattering communication principle which is illustrated in FIG. 1. When a tag 10 communicates with an RFID reader 11, it modulates the received signal 12 and reflects a portion of it 13 back to the reader. A typical passive tag consists of an antenna connected to an application specific microchip. When wirelessly interrogated by an RFID transceiver, or reader, the RFID tag antenna receives power and RF signals from the RFID reader and provides them to the chip. The chip processes the signals and sends the requested data back to the RFID reader. The backscattered signal is modulated according to the transmitted data. The highest operation frequency and read-out distance of RFID are limited by the rectified power for the integrated circuit (IC) and are a few GHz and 5-10 m, respectively.
RFID is mostly used for identification. RFID tags are equipped with a rewritable memory, which enables the reusability features of RFID tags, but they are not useful for measuring external quantities. RFID has also been shown to be suitable for sensing by equipping an RFID tag with an external sensor and digital logic to read the external sensor. The advantage of this approach is that it would use a generic sensor element and thus would be well suited for a very broad range of applications. In this approach, however, an additional A/D converter and digital circuitry has to be included to the tag in order to enable sensor read-out. Increased power consumption due to the additional electronics reduces the read-out range significantly (e.g. from 5 m to 0.3 m with an 8-bit A/D converter). An additional sensor element further increases power consumption. Implementation considerations of the A/D converter and additional digital circuits are discussed in [1]: Chapter 9 “Smart RFID Tags”, in the book “Development and Implementation of RFID Technology”, ISBN 978-3-902613-54-7, February 2009, I-Tech, Vienna, Austria. http://www.intechopen.com/books/development_and_implementation_of_rfid_technology.
US2013/0099897 discloses an RFID reader, an RFID chip, and an antenna electrically coupled to the RFID chip and configured to receive signals from and transmit signals to the RFID reader. The RFID chip is provided with an electrical interface to a sensing material. The RFID chip is configured to modulate a signal received from a reader and to drive the sensing material with the modulated signal. The sensing material has a variable electrical property, such that the backscattered modulated signal will change according to the condition of the sensing material. Regardless of the nature of the sensing material, it interacts with the modulated signal from the RFID chip and returns the signal to the RFID chip. The returned signal is passed from the RFID chip to the antenna via the backscatter modulator and then transmitted back to the RFID reader. Alternatively, the signal processed by the sensing material is used to modulate the input impedance of the RFID chip, with a signal from the RFID chip being backscattered to the RFID reader by the antenna to determine the condition of the sensing material.
Chen et al, Coupling Passive Sensors to UHF RFID Tags, Radio and Wireless Symposium (RWS), 2012 IEEE, 15-18 Jan. 2012, Santa Clara, 255-258, explores the possibility of coupling passive sensor data to existing UHF RFID tags without designing a new tag ASIC. The existing UHF RFID system can be used to convey additional data by overlaying a coupling loop on the tag antenna and modulating vector backscatter. The impedance of the passive sensor carrying the sensor data influences the value of amplitude and phase of the backscattering. For the transmission of the passive sensor data, the load of the passive sensor coupling module is switched between these three loads to provide the connection to one of the two reference impedances or the passive sensor. With two reference impedances, the impedance of the passive sensor is determined.
Guerin et al., A temperature and gas sensor integrated on a 915 MHz RFID UHF tag, Wireless Information Technology and Systems (IC-WITS), 2010 IEEE International Conference, Honolulu, Aug. 28 2010-Sep. 3 2010 discloses a passive wireless sensor utilizing the modulated backscattering principle. The modulation signal is generated by a voltage-controlled oscillator whose control voltage and thereby the output frequency is arranged to change in function of the sensor value.
Co-pending PCT/FI2013/051214 discloses passive wireless sensor design that enables a radically increased reading distance of passive wireless sensors. The modulation signal is generated by an oscillator that includes a sensing element as a part of an oscillating circuit, such that the modulation frequency is dependent on a sensed value of the sensing element. The RFID sensor operates in a similar way as a normal UHF RFID tag, except that the modulation frequency of the tag carries sensor information. According to the ISO/IEC 18000-6C standard, the modulation frequency of the tag is set by the reader device at the beginning of the communication, and it can be from 40 kHz to 640 kHz. The modulation frequency of the tag can vary ±4 . . . 22% from the initially set frequency depending on the frequency and operating conditions. This enables coding sensor information to the modulation frequency. This type of RFID sensor can be realized by modifying the internal oscillator of an RFID chip in such a way that its oscillation frequency tunes with the measured quantity. This can be done by loading the oscillator with an inductive, capacitive or resistive sensor element. An advantage of this principle is that generic sensor elements, such as MEMS sensors, can be used because the sensor is read out at low kHz frequencies. The sensor value is translated into a frequency of modulated analog signal which can be generated without an energy consuming AD conversion and with minimum number of extra component. The sensing element does not necessarily increase the power consumption of the tag, and the principle therefore offers a long read-out distance. The sensor circuitry is also fully isolated from the antenna, making the concept insensitive to antenna proximity effects. This concept can potentially be made compatible with the existing RFID systems, and many sensing elements can be incorporated into one RFID tag with digitally controlled switches. The sensing elements may be arranged so that the oscillation frequency is affected by only one element at a time.
In this type of RFID sensor, it is important that the RFID sensor can be made sensitive enough such that even a small change in the sensed property changes the modulation frequency so much that the change can be detected. The electrical properties of the sensor element are often given, and the oscillation frequency is typically constrained by frequency regulations, standards, or reader hardware. Therefore, the values of the oscillator components must be selected or adjusted such that the oscillation is obtained at the correct frequency and the sensitivity is at the desired level. This may require large components which may increase the size, the losses and the power consumption of the oscillator, and may not be practical task for all type of sensor elements. The designing must be done separately for each type of sensor element.